The invention relates to a device for producing or amplifying coherent radiation comprising at least a first body provided with at least a first layer of a material suitable for producing or amplifying such a radiation having at least an active region, in which in the operative state population inversion is obtained by means of electron injection.
Such a device can be used as a semiconductor laser, for example, when the first layer is a suitably chosen layer of semiconductor material. The active region is then situated within a resonator, which is constituted, for example, by two parallel cleavage surfaces of a crystal to which the first layer of semiconductor material belongs. The arrangement may also be used, however, as a selective amplifier of coherent radiation, in which event no reflection members are utilized.
The operation of such a laser or travelling-wave amplifier requires that population inversion occurs in the active layer. Population inversion is obtained if a higher energy level is occupied more strongly than a lower energy level. In semiconductor lasers, this inversion is generally produced by means of electric current; in particular semiconductor lasers are generally constructed as injection lasers, whereby the current through a pn junction provides the required injection. The pn junction may then extend between two parts of the active layer or between the active layer and an adjoining passive layer. Use is generally made of doped gallium arsenide (GaAs) as a material for the active layer of such an injection laser. The wave length of the electromagnetic radiation emitted by such a laser is approximately 900 nm.
For various reasons it is desirable to manufacture lasers of small dimensions which emit radiation of a shorter wave length. For example, when storing information in image and sound carriers (VLP, DOR, Compact Disc), the required surface area for one information bit is inversely proportional to the square of the wave length. Consequently, when the wave length is halved, the possibility is obtained of quadrupling the information density. An additional advantage is that at shorter wave lengths simpler optics may be used.
In order to be able to manufacture semiconductor lasers having a shorter wavelength, use could be made of semiconductor materials having a larger forbidden band-gap than that of gallium arsenide. Materials having a larger forbidden band gap are, for example, zinc oxide (ZnO), cadmium zinc sulphide (CdZnS), zinc sulphide (ZnS) and cadmium zinc selenide (CdZnSe).
A device of the kind mentioned in the opening paragraph is known from the article "Temperature-Induced Wavelength Shift of Electron Beam Pumped Lasers from CdSe, CdS and ZnO" by I. M. Hvam, published in Physical Review B, Volume 4, No. 12, p. 4459-4464 Dec. 15.sup.th 1971). In this article, a measuring arrangement is shown by which laser properties of II-VI compounds are examined by bombarding them with electrons from an electron gun. In the article, it is demonstrated that, for example, in zinc oxide (ZnO) and cadmium sulphide (CdS) stimulated emission with associated wavelengths of approximately 400 nm and approximately 500 nm, respectively, is obtained.
Therefore, zinc oxide and cadmium sulphide would be very suitable for the manufacture of a semiconductor laser for short wavelengths if it had not proved inpossible hitherto to provide in semiconductor bodies manufactured from these materials a pn junction by which, by means of an electric current, injection and population inversion can be obtained in an active layer. The apparatus for experimental use described in the aforementioned article is of course too voluminous and too expensive for incorporation in VLP systems etc.